An adhesive is required to adhere to an adherend so that it would hardly be peeled. On the other hand, a pressure sensitive adhesive is formed into a shape of a tape or the like and is required to exhibit good adhesion immediately upon application. At the same time, the pressure sensitive adhesive is required to have removability so that it can be peeled without leaving an adhesive deposit. While permanent adhesion is required for an adhesive, a pressure sensitive adhesive is required to have both temporary adhesion and removability. Thus, although an adhesive and a pressure sensitive adhesive are alike, they are fundamentally different in the required properties.
As conventional pressure sensitive adhesives, an acrylic pressure sensitive adhesive, a rubber pressure sensitive adhesive, a silicone pressure sensitive adhesive, an urethane pressure sensitive adhesive and an oxyalkylene pressure sensitive adhesive are known. Particularly, it has been a recent tendency that an acrylic pressure sensitive adhesive is used in a wide range of applications from a strongly adhesive pressure sensitive adhesive having a strong adhesive strength to a weakly adhesive pressure sensitive adhesive having a weak adhesive strength. However, an acrylic pressure sensitive adhesive has a problem of an odor or dermal irritation in a case where an acrylic monomer remains in the pressure sensitive adhesive. Further, an acrylic pressure sensitive adhesive is likely to undergo a change with time after being applied to an adherend, whereby the adhesive strength tends to increase, or migration tends to be high. Accordingly, there has been a problem such that an adhesive deposit is likely to remain on the adherend, and the removability tends to be inadequate.
In the case of a rubber pressure sensitive adhesive, it is necessary to add a low molecular weight plasticizer to adjust the adhesive performance or for handling efficiency. Therefore, there is a problem such that upon expiration of a long period of time, the low molecular weight plasticizer migrates to the surface, thus bringing about a remarkable deterioration of the performance.
A silicone pressure sensitive adhesive is excellent in heat resistance. However, it is expensive and is used only for special applications.
An urethane pressure sensitive adhesive has a characteristic such that it is excellent in stability with little change with time (Patent Document 1). However, its production process tends to be long, and as compared with an acrylic pressure sensitive adhesive, it tends to be expensive. Further, as the production process is long, there has been a problem that fluctuation in the quality is likely to result, or the process management tends to be cumbersome.
An oxyalkylene pressure sensitive adhesive has a characteristic such that it can be applied without using an organic solvent (Patent Documents 2 and 3). However, bleeding of a tackifier may sometimes occur, and there has been a problem from the viewpoint of the stability of the adhesive strength for a long period of time.
In recent years, a protective sheet or a protective tape has been frequently used during the production of electric components or electronic materials. The purpose is to protect such components or materials from scratching or dusts in the process for transportation, storage, etc. Particularly, in the production of electronic components or optical materials, it is necessary to completely prevent fine dusts from attaching to a product during its production. Because, dusts will cause contamination and thus cause product failure. As such a protective sheet or a protective tape, a pressure sensitive adhesive sheet or a pressure sensitive adhesive tape provided with a pressure sensitive adhesive layer having a low adhesive strength, is employed.    Patent Document 1: JP-A-2003-12751    Patent Document 2: WO2005/73333    Patent Document 3: WO2005/73334